Gov. Chris Christie of New Jersey on Thursday appointed Jeffrey S. Chiesa, the state’s attorney general, to temporarily fill the Senate seat left vacant by the death of Frank R. Lautenberg.

A special election to fill the seat for the remainder of Mr. Lautenberg’s term will be held in October, and Mr. Christie’s announcement came as Democrats and Republicans across the state scrambled to line up support, raise money and secure enough signatures ahead of the deadline on Monday to compete in the August primary.

Mr. Chiesa, a Republican, does not plan to seek the seat himself, Mr. Christie said, opening up the field for others in the party. The governor revealed his decision at an afternoon news conference in Trenton.

“It was unexpected for sure,” Mr. Chiesa said. He declined to detail his views on specific policy issues, including immigration, saying only that he wanted to be sure the nation’s borders were secure. He described himself as a “conservative Republican.”

“I need to learn about the issues before I can make any meaningful judgments,” he said.

He offered a flash of his laconic humor when, after being read a long litany of issues he may have to confront during his short tenure, he replied, “Oh, is that it?”

Mr. Chiesa served as chief counsel to Mr. Christie from January 2010 through December 2011, when he was nominated to be attorney general. Before that, he headed Mr. Christie’s transition team after his election, and the two also worked together at the United States attorney’s office.

Senator Lautenberg, who was in his fifth term, died on Monday at age 89. Mr. Christie first reached out to Mr. Chiesa about filling the seat that night, and he noted that the two had been confidants going back more than two decades.

“There are very few people in my life I know better than Jeff,” the governor said. “You won’t find anybody, I think, who will have something bad to say about Jeff.”

Mr. Chiesa, 47, lives in Branchburg, N.J., with his wife and two children. As attorney general, he aggressively went after those suspected of trying to exploit consumers following Hurricane Sandy, filing fraud charges against 27 businesses in the state. He began an ambitious gun-buyback program that took 10,000 weapons off the streets, formed a division to help combat human trafficking and, in 2012, prosecuted current and former public officials in 40 corruption cases.

Matthew Hale, an associate professor at Seton Hall University who closely follows New Jersey politics, said that aside from an assumption that Mr. Chiesa would hold a tough line on law and order issues, it was hard to know how he might vote on many matters in Washington.

“There is just not a huge amount of information we have on his positions,” Dr. Hale said.

Still, he said, Mr. Chiesa is a safe choice for the governor. “This is someone that Chris Christie knows really, really well, trusts a great deal, and will not be surprised by,” he said.

Mr. Christie said he would appoint a new attorney general on Monday.

The decision on the interim senator promised to shift the dynamics of the race, especially for Republicans.

On the Democratic side, Representative Rush Holt, who has represented central New Jersey for eight terms in Congress, on Thursday announced his intention to compete for the seat. In an e-mail to supporters, Mr. Holt said he was “the best candidate to continue the passionate advocacy for progressive values” shown by Mr. Lautenberg.

Mr. Holt is likely to face Mayor Cory A. Booker of Newark, a prodigious fund-raiser who has become one of the state’s most visible politicians. He had been planning to run for the Senate even before Mr. Lautenberg died. Another Democrat, Representative Frank Pallone Jr., who has served 13 terms, is also expected to run. He has millions of dollars in campaign money available for the race.

The Republican primary picture remains more uncertain. Democrats in the state outnumber Republicans by 700,000 among registered voters.

Steven M. Lonegan, who competed with Mr. Christie in the 2009 primary for governor, has announced he will run for the seat. Other names that have been mentioned as possible candidates include Lt. Gov. Kim Guadagno and former State Senator Bill Baroni, who is the deputy executive director of the Port Authority of New York and New Jersey.

The day after Mr. Lautenberg died, Mr. Christie announced plans to hold a special Senate election on Oct. 16, a Wednesday, with primary contests on Aug. 13. His move to have the vote separate from the Nov. 5 general election, when he is on the ballot for a second term, drew fierce criticism from Democrats, as well as some Republicans, who said it was an unnecessary expenditure. (The arrangement is expected to cost an additional $24 million.)

Mr. Christie, at the news conference on Thursday, once again defended his move on scheduling the Senate vote.

“There was no perfect decision,” he said, adding that the state’s statutes on the subject of special elections are vague and open to interpretation, which could lead to court challenges.

“As far as the ramifications politically, that is for everyone else to decide,” he said.

The practical impact has been to force potential candidates to decide quickly whether they want to compete for the seat and then adjust their tactics to navigate a compressed campaign schedule.

Republicans and Democrats hoping to run must file petitions by Monday afternoon and must include 1,000 signatures from supporters.

Correction: June 6, 2013

An earlier version of this article misspelled the surname of the chairman of the Port Authority of New York and New Jersey. He is David Samson, not Sampson.

A version of this article appears in print on June 7, 2013, on page A21 of the New York edition with the headline: Christie Puts Attorney General in Senate. Order Reprints|Today's Paper|Subscribe